Compare the distribution of pulmonary blood flow to the distribution of ventilation.
6.
List and define the four categories of hypoxia and the six causes of hypoxic hypoxia.
7.
Distinguish the causes of hypoxic hypoxia on the basis of the response to 100% O2.
8.
Describe the two causes of hypercapnia.
he major determinant of normal gas exchange and thus the level of PO2 and PCO2 in blood is the relationship between ventilation (V̇ ) and perfusion (Q̇ ). This relationship is called the ventilation/perfusion (V̇ /Q̇ ) ratio.
Ventilation is the process by which air moves in and out of the lungs. The incoming air is composed of a volume that fills the conducting airways (dead space ventilation) and a portion that fills the alveoli (alveolar ventilation). Minute (or total) ventilation (V̇ E) is the volume of air that enters or leaves the lung per minute:
Equation 23.1 where f is the frequency or number of breaths per minute and VT (also known as TV) is the tidal volume, or volume of air inspired (or exhaled) per breath. Tidal volume varies with age, sex, body position, and metabolic activity. In an average-sized adult at rest, tidal volume is 500 mL. In children, it is 3 to 5 mL/kg.
Dead Space Ventilation: Anatomical and Physiological
Dead space ventilation is ventilation to airways that do not participate in gas exchange. There are two types of dead space: anatomical dead space and physiological dead space. Anatomical dead space (VD) is composed of the volume of gas that fills the conducting airways:
Equation 23.2 where V refers to volume and the subscripts T, D, and A refer to tidal, dead space, and alveolar. A “dot” above V denotes a volume per unit of time (n):
Equation 23.3
Equation 23.4 where V̇ E is the total volume of gas in liters expelled from the lungs per minute (also called exhaled minute volume), V̇ D is the dead space ventilation per minute, and V̇ A is alveolar ventilation per minute.
In a healthy adult, the volume of gas contained in the conducting airways at functional residual capacity (FRC) is approximately 100 to 200 mL, in comparison with the 3 L of gas in an entire lung. The ratio of the volume of the conducting airways (dead space) to tidal volume represents the fraction of each breath that is “wasted” in filling the conducting airways. This volume is related to tidal volume (VT) and to exhaled minute ventilation (V̇ E) in the following way:
Equation 23.5
CHAPTER 23 Ventilation, Perfusion, and Ventilation/Perfusion Relationships
If the dead space volume is 150 mL and tidal volume increases from 500 to 600 mL for the same exhaled minute ventilation, what is the effect on dead space ventilation?
= 0 3. × VE and, similarly,
Increasing tidal volume is an effective way to increase alveolar ventilation (and thus normal blood gas values), as might occur during exercise or periods of stress. As tidal volume increases, the fraction of the dead space ventilation decreases for the same exhaled minute ventilation.
Dead space ventilation (VD) varies inversely with tidal volume (VT). The larger the tidal volume, the smaller the proportion of dead space ventilation. Normally, VD/VT is 20% to 30% of exhaled minute ventilation. Changes in dead space are important contributors to work of breathing. If the dead space increases, the individual must inspire a larger tidal volume to maintain normal levels of blood gases. This adds to the work of breathing and can contribute to respiratory muscle fatigue and respiratory failure. If metabolic demands increase (e.g., during exercise or with fever), individuals with lung disease may not be able to increase tidal volume sufficiently.